O DEPÓSITO BAHIA: UM POSSÍVEL EXEMPLO DE DEPÓSITO DE SULFETO VULCANOGÊNICO TIPO BESSHI ARQUEANO EM CARAJÁS

Abstract

The Bahia ore deposit is hosted by the Archean Igarape Bahia Group which is one of the volcano-sedimentary sequences that have filled the Carajas basin (SE Para state). This group consists of basic metavolcanic, metapyroclastic and clastic metasedimentary rocks, besides banded iron formations and breccias. These rocks are cut by basic dikes and have been altered by hydrothermal fluids to mineral assemblages compatible with the greenschist facies metamorphic conditions. The Bahia deposit is formed by the Acampamento Norte, Acampamento Sul, Furo Trinta and Alemao ore bodies. The first three ones show disseminated mineralization that is hosted mainly by breccias. The last one was recently discovered and is partially made up of massive sulfide lenses. The breccias occur in the verticalized contact between the basic metavolcanic and the clastic metasedimentary rocks and usually display gradational boundaries with the host rocks. The lithoclasts are either from basic metavolcanic rocks or banded iron formations. Their shapes vary form angular to subangular. The matrix is composed of chlorite, siderite, chalcopyrite, quartz, magnetite, tourmaline and calcite. The matrix/clasts ratio is highly variable producing clast-supported to matrix-supported breccias, some of which showing slight foliation and orientation of the lithoclasts. To these breccias it is suggested a phreatic origin whose fragments were reworked and transported into deeper zones as debris flow. Compared to the host rocks, Cu and Au are more enriched in the breccias, in which the sulfides also occur in veins and pods together with quartz and/or siderite. Chalcopyrite and the pyrite are the main sulfides, but in the massive magnetite and sulfide-rich beds bornite is also present. Magnetite is an abundant phase found not only in banded iron formation clasts, but also in the breccia matrix as disseminations or in massive beds. Chloritization, carbonation, magnetitization and sulfidation are the most important types of hydrothermal alteration, although silicification and tourmalinization may also occur. Pervasive Chloritization has affected ali rocks in greater or lesser degree. Microthermometric studies in quartz crystals present in the breccias revealed aqueous fluids chemically represented by the H 2 O-NaCl-CaCl 2 system with variable salinities (5.5-41.5 weight % eq. NaCl). It was also observed pure CO 2 inclusions. The more frequent homogenization temperature ranges were 110-140°C and 150-225°C respectively for the two-phase and three-phase aqueous inclusions. The aqueous fluids have been assumed to be convectively driven sea water whereas the carbonic fluids were probably mantle-derived. The most saline aqueous fluids (>23% wt equiv. NaCl) are believed to have had a magmatic contribution. The geological characteristics of the Bahia deposit allow it to be interpreted as a Besshi-type volcanogenic deposit in which uranium and rare-earth elements, that occur in anomalous concentrations, have been introduced by later events.